What are the differences between these two actions? Can the P-14 action be used to build
a big bore such as a 458 Lott?

Slightly differant bolt, magazine box set up for rimmed cartridges on the 14, a 303 barrel instead of a 30-06 barrel, some slight differances in sights. Yes, you can build a Lott on either a 14 or a 17. The 14 has advantages when it comes to larger cases (Rigby, Wby, Gibbs, et al) because of the bolt face design. - Dan

If you built a Lott (.532" bolt face) on a P14, there will be some slip on where the case head contacts the bolt but it will be ok.

The problem is opening the rails, lengthening the mag box cut, building a new mag box, building a new follower, and getting it all to feed perfectly.